In pulsed radar systems, the pulses generated by the transmitter are amplified by a power amplifier, and then applied to one or more radiating or antenna elements for propagation. The time duration of each of these pulses may be substantially less than the space between them. Thus, in order to conserve power, the amplifier may be switched on or off in synchronism with the generation of each RF pulse. In the past, power amplifiers were pulsed on and off by utilizing separate pulsing circuitry to pulse the grid of an output traveling waveguide tube. Since this tube tended to have a high gain, typically forty dB, for example, such pulsing circuit was relatively insignificant in contributing to the power used by the transmitter.
However, many present day radar systems utilize solid state transistor power amplifiers, which are well known in the art, and any pulsing circuit which turns such amplifiers on and off in synchronism with the generated RF pulses may contribute substantially to the overall power requirements of the transmitter. Also, since it is advantageous to mount such solid state power amplifiers in the antenna to minimize combining losses, any pulsing circuitry requires additional conductors between the antenna and the transmitter. Thus, it is desirable to eliminate such pulsing circuitry with its attendant power requirements and additional conductors.
Conventional solid state power amplifiers, of course, may be pulsed on and off in response to each of the generated RF pulses without the separte pulsing circuitry by using high power field effect transistors, and operating in the class B or class C region; and it has been determined that such pulsing may be accomplished with substantial efficiency in the low and intermediate frequencies, such as up to about three gigahertz. For example, a solid state amplifier using a one micron gate field effect transistor provided a 10dB gain with a one watt output at 21/2 gigahertz, which amounted to an increased power added efficiency of approximately 35%. Power added efficiency herein is defined as that percentage of DC drain power input to the transistor which is actually output in the form RF power or RF Power Out-RF Power In/DC Drain Power=Power added efficiency.
However, in radar systems where the RF pulses are of afrequency that may be substantially higher than three gigahertz, such as in X band radar systems, the power added efficiency of such class B operation of the solid state amplifiers using medium to high power field effect transistors, may be substantially reduced. In one example, at a frequency of 8 gigahertz, a power amplifier utilizing a 1/4 watt gallium arsenide field effect transistor, which produced approximately a 4.1dB gain, provided only a 21% power added efficiency. Such decreased efficiency at the higher frequencies is believed due to various solid state technology problems, gate circuit resistance, for example, which do not permit low pinch-off voltages, high power, and gain.
Therefore, it is desirable to be able to provide power amplifiers for pulsed radar transmitters that turn on and off in response to the presence and absence of the generated RF pulse, using power field effect transistors that operate at class B or below; and which provide efficiency not only at the lower and intermediate frequencies, but at the higher X band frequencies as well.